586 Mr. mann’s Treatife 
rapidity of the back-waters a 61s upon the ftream, which 
will always be as far as till this force is gradually, and at 
laft wholly, deftroyed by the fuperior gravitation of the 
fuper-incumbent waters in the ftream. Something of the 
fame kind happens when a very great additional weight 
comes fuddenly upon the furface of a river or canal ; for 
inftance, by the launching of a fhip or of feveral boats 
together uponit. Thefe caufes increafe the velocity of 
the water in the lower parts of the bed, and moreover 
retard its motion at the furface, which effe61 may pro- 
perly be called making tbe river's bottom move. For 
the fame reafon, the increafe of weight of the waters 
in a fudden flood, as well as the increafe of their impul- 
live force, muft contribute to produce this effedt, and, by 
increafing the motion in the bottom of the river, may 
hinder, for fome fpaee of time, the ftream from fenfibly 
riling in the bed. 
33. All obftacles whatever in the bed of a river or 
canal, fuch as rocks, trunks of trees, banks of fand and 
mud, 8cc. muft neceffarily hinder proportionably the free 
running off of the water; for it is evident, from what we 
have faid, that the waters fo far back from thefe obfta- 
cles, until the horizontal level of the bottom of the bed 
becomes higher than the top of the obftacles, muft be 
intirely kept up and hindered from running off in pro- 
portion 
